1. Field of the Invention
The present invention relates to follow-up control. The present invention can be applied to, for example, a control system for performing a focusing control or a tracking control of a light spot in an optical disk drive.
2. Description of the Background Art
First, the focusing or tracking control in a general optical disk drive will be briefly described. An optical disk is chucked by a spindle motor, and the rotation of the spindle motor is controlled so that the optical disk rotates at a predetermined linear velocity.
To realize accurate recording and reproduction in an optical disk drive, a light spot from an optical pickup has to be converged and positioned with high precision onto a recording track spirally formed in the face of an optical disk. Since it is difficult to make an axis of a center hole of an optical disk and the rotary axis of a spindle motor perfectly coincide with each other, an eccentricity of hundreds μm occurs in the rotation of the optical disk. In relation to mechanical machinery precision of the surface of an optical disk, an face runout of hundreds μm occurs in the rotation of an optical disk. In some cases, physical distortions peculiar to an optical disk, such as eccentricity and runout are generically referred to as a “disk physical distortion” hereinafter.
When eccentricity occurs in rotation of an optical disk, recording tracks of the optical disk meander. Consequently, it is necessary to perform a tracking control for following a recording track. It is also necessary to perform a focusing control for making a light spot follow and converge to a recording track in accordance with a runout which occurs in association with rotation of an optical disk. The position control in this specification is a concept including both the tracking and focusing controls. Such a position control can be realized by following the position of a movable part of an actuator to which an objective lens is attached.
A1. First Conventional Art
FIG. 43 is a block diagram showing the configuration of a conventional control system 301 for controlling a position to be controlled, which is, for example, a position control system for performing a focusing or tracking control on a light spot in a vehicle-mounted or portable optical disk drive.
In the control system 301, a position x to be controlled follows a follow target position d_p. A subtracter 1 subtracts the position x to be controlled from the follow target position d_p to thereby obtain a position error d_p_e. A control for converging the position error d_p_e on almost zero is performed. For example, the position x to be controlled is set to zero at a mechanical midpoint.
In the case where the control system 301 is applied to an optical disk drive, there is no means for directly detecting each of the position x to be detected and the follow target position d_p. By electrically computing a signal detected by optical means of an optical pickup, the position error d_p_e is just obtained as an electrical signal. An actuator is not usually provided with a sensor for detecting the speed. Therefore, a unity feedback system in which the position x to be controlled and the follow target position d_p are inputted to the schematically-shown subtracter 1 is constructed.
A phase compensating block 2 performs phase compensation on the position error d_p_e to thereby obtain a position control signal p_cont. The position control signal p_cont is an electrical amount which is, for example, a voltage. A electricity-to-force converting block 3 converts the position control signal p_cont to a force and outputs a position control force z2. To an actuator for supplying the position x to be controlled, not only the position control force z2 but also an acceleration disturbance force z_acc based on an acceleration disturbance Di_acc to be added to the object to be controlled are also added. This addition is expressed by an adder 5. Generation of the acceleration disturbance force z_acc is expressed by a multiplier 9 for multiplying the acceleration disturbance Di_acc with a mass “m” of the object to be controlled.
An output from the adder 5, that is, a result of addition of the acceleration disturbance force z_acc and the position control force z2 is supplied as an external force z1 to an actuator mechanism block 4. The actuator mechanism block 4 converts the external force z1 to the position x to be controlled. In other words, the position x to be controlled is the position of the movable part of the actuator when the external force z1 is applied to the actuator.
The position x to be controlled is, for example, the position of the movable part of the actuator of an optical pickup in an optical disk drive. The follow target position d_p is a target value of the position x to be controlled, that is, a target value of the position of the movable part of the actuator. If the control system 301 is, for example, a control system for performing a focusing control, the position error d_p_e is a so-called “focus error signal”. If the control system 301 is, for example, a control system for performing a tracking control, the position error d_p_e is a so-called “track error signal”.
The phase compensating block 2 performs phase compensation on the position error d_p_e on the basis of a phase compensation characteristic F(s). The phase compensation characteristic F(s) is a characteristic including a stabilization compensation for assuring a phase margin by advancing a phase around the crossover frequency of the control system 301 and a low-bandwidth compensation characteristic for partly increasing a gain in a bandwidth lower than the crossover frequency. By performing the stabilization and low-bandwidth compensation, the position control signal p_cont is obtained.
A electricity-to-force converting characteristic H(s) corresponds to the characteristic of the electricity-to-force converting block 3 and is defined as a characteristic including a driver gain and a current-to-force characteristic indicative of the relation between a current and a force in a driving magnetic circuit of the actuator.
An actuator mechanism characteristic G(s) corresponds to the characteristic of the actuator mechanism block 4 and is expressed by, for example, a secondary system. Concretely, in an actuator expressed by a model using a mass, a spring, and a dashpot, the characteristic indicates the relation between a force applied to the actuator and the position of the movable part of the actuator.
FIGS. 44A and 44B are graphs showing an example of open-loop characteristic in the control system 301. FIG. 44A shows a gain characteristic, and FIG. 44B shows a phase characteristic. As shown in FIGS. 44A and 44B, the gain at 8 Hz or lower which is around the maximum rotation frequency of a CD player is increased and, further, the phase of a bandwidth around the crossover frequency (1 kHz) at which the gain is 0 dB is advanced, thereby assuring a predetermined phase margin.
Open-loop characteristic in the case of applying the control system 301 to a CD player is subjected to loop shaping so that a gain of 60 dB or higher in a low bandwidth, a control bandwidth of 1k to 3 kHz, a phase margin of 40 to 60 degrees, and a gain margin of 10 to 20 dB can be assured. The open-loop characteristic can be realized by properly designing the actuator mechanism characteristic G(s) and the phase compensation characteristic F(s).
FIGS. 45A and 45B are graphs showing the operation of the control system 301 for performing the tracking control, having the open-loop characteristic illustrated in FIGS. 44A and 44B. A case where the acceleration disturbance Di_acc is not added to a control target, that is, a case where Di_acc=0 is shown. FIG. 45A shows a state of a change in a follow target position d_p, that is an eccentricity amount of an optical disk. It is assumed here that the follow target position d_p changes at 8 Hz around the maximum rotation frequency of a CD player, and an amplitude is 140 μmpp of a specification limit. That is, FIG. 45A shows a value of the follow target position d_p when a CD player operates under the worst conditions.
FIG. 45B shows a state of a change in the position error d_p_e in the case where the follow target position d_p changes as shown in FIG. 45A, that is, a value in the case where the control system 301 is in a steady state. Hereinafter, the position error d_p_e at the time the control system enters in the steady state will be called a “residual error”.
As shown in FIGS. 45A and 45B, even in the case where a CD player operates under the worst conditions, if the influence of the acceleration disturbance Di_acc can be ignored, the residual error can be suppressed up to about 0.09 μmpp. Generally, when the residual error becomes equal to or larger than 0.1 μmpp, a reproduction signal deteriorates. If the control system 301 has the open-loop characteristic shown in FIGS. 44A and 44B, the reproduction signal does not deteriorate even under the worst conditions, so that normal reproduction can be assured.
In reality, however, vibration or the like is applied to a vehicle-mounted or portable optical disk drive. For example, to a portable computer-mounted ODD device, a portable CD player, and a portable MD player, external vibration created by carriage by the user or external vibration created when the device is attached to a vehicle, a table, or the like is applied to the actuator of an optical pickup.
As shown in FIG. 43, not only the position control force z2 but also the acceleration disturbance force z_acc based on the acceleration disturbance Di_acc are inputted to the actuator mechanism block 4. An influence of the acceleration disturbance Di_acc is exerted, and the position x to be controlled is largely deviated from the follow target position d_p and, as a result, the position error d_p_e increases.
FIGS. 46A and 46B are graphs showing the sensitivity characteristic of the control system 301 having the open-loop characteristic illustrated in FIGS. 44A and 44B. FIG. 46A shows the gain characteristic, and FIG. 46B illustrates the phase characteristic.
FIGS. 47A and 47B are graphs showing the operation of the control system 301 having the open-loop characteristic illustrated in FIGS. 44A and 44B. FIG. 47A shows the waveform of the acceleration disturbance Di_acc having a frequency of 30 Hz and an amplitude of 5 Gpp. FIG. 47B shows a residual error occurring when the acceleration disturbance Di_acc illustrated in FIG. 47A is given. In the graphs, the follow target position d_p is set to zero.
When the acceleration disturbance Di_acc is given to the control system 301, as shown in FIG. 47B, the residual error becomes 1.5 μmpp. Such a magnitude of the residual error causes deterioration in a reproduction signal of an optical disk, and a data error rate of information reading increases. In the case where the optical disk drive is a recorder, due to an influence of the acceleration disturbance Di_acc, information may not be properly recorded.
A2. Second Conventional Art
In an actual optical disk drive, there is a case that an optical disk having a defect such as a blemish or dirt is reproduced or recorded.
In a region where a defect occurs (hereinafter, provisionally referred to as a “defective region”) in the surface of an optical disk, light is not normally reflected by the disk and optical information is lacked. When the follow-up control using the position error d_p_e is performed in the defective region, the actual difference between the position x to be controlled and the target value d_p becomes large, and a problem such that a focus error occurs in the focusing control and retracing to a neighboring track in the tracking control occurs.
To deal with such a problem, it is desirable to configure a control system by using a function (hereinafter, provisionally referred to as a “defect compensating function”) enabling a position control to be performed continuously even after actuator shifts from a region where no defect occurs (hereinafter, provisionally referred to as a “defect-free region”) to the defective region.
FIG. 48 is a block diagram showing the configuration of the conventional control system 302 having such a defect compensating function. The control system 302 can be employed as a position control system for performing a light spot focusing or tracking control in an optical disk drive.
As shown in FIG. 48, the control system 302 has a configuration such that a problem preventing block 103 for obtaining a defect compensating function is interposed between the phase compensating block 2 of the control system 301 and the electricity-to-force converting block 3.
The problem preventing block 103 has a selector 102 and first and second paths selected by the selector 102. The selector 102 outputs a signal sent via the first or second path as an actuator control signal cont in accordance with a first event or a second event complementary to the first event. In the control system 302, different from the control system 301, the actuator control signal cont inputted to the electricity-to-force converting block 3 does not always coincide with a position control signal p_cont.
The first and second events correspond to, for example, a state where the actuator encounters the defect-free region and a state where the actuator encounters the defect region, respectively. The first and second events are identified by a defect detection signal DEFECT separately supplied and correspond to, for example, signal levels “L” and “H” of the defect detection signal DEFECT, respectively.
The defect detection signal DEFECT goes high “H” or low “L” in accordance with whether the actuator encounters the defect region (this period is provisionally referred to as a “defect period”) or the defect-free region (this period is provisionally referred to as a “defect-free period”). The defect detection signal DEFECT is generated, for example, on the basis of an amount of light reflected by the disk.
Via the first path, the position control signal p_cont outputted from the phase compensating block 2 is inputted as it is to the selector 102. The second path is provided with a low-pass filter 100 and a sample and hold circuit 101. The low-pass filter 100 extracts and outputs low-frequency components including disk rotation frequency from the position control signal p_cont. An output of the low-pass filter 100 is supplied to the sample and hold circuit 101. The sample and hold circuit 101 performs the sample and hold operation at a timing when the signal level of the defect detection signal DEFECT changes from “L” to “H”.
When the actuator encounters the defect-free region, the first path is selected. Also in the control system 302, in a manner similar to the control system 301, the position control signal p_cont is substantially inputted to the electricity-to-force conversion block 3. However, when the actuator encounters the defect region, the second path is selected. In the control system 302, therefore, the position control signal p_cont at the time of transition from the first event to the second event just before the present second event starts, that is, at the time of transition from the defect-free region to the defect region (hereinafter, provisionally referred to as “entrance of a defect”) is inputted as the actuator control signal cont to the electricity-to-force converting block 3. Moreover, from the position control signal p_cont, the low frequency components are extracted as described above.
Since an input of the sample and hold circuit 101 is an output of the low-pass filter 100, the position control signal p_cont without the influence of high-frequency noise added can be sampled and held. An output of such a sample and hold circuit 101 is, so to speak, an average value of the position control signals p_cont in the defect-free region and can be regarded as a disk physical distortion correction signal for correcting a physical distortion of a disk. By employing the disk physical distortion correction signal as the actuator control signal cont in the defect region, the continuity of inputs to the electricity-to-force converting block 3 is achieved and the defect compensating function is realized.
As described above, as the technique using the low frequency components of an output of the phase compensating block in the defect-free period, for example, a technique disclosed in Japanese Patent Application Laid-Open No. 11-250478 can be used.
To realize stable and reliable defect compensation, it is a necessary condition that leading to a phase control loop is normally done at the point of transition from the defect region to the defect-free region (hereinafter, provisionally referred to as an “exit of a defect”).
Conditions of normally leading to the position control loop are that the position error d_p_e at the leading operation is around zero, and the difference between a time differential x′ of the position x to be controlled indicative of the speed of a control target and a time differential d_p′ of the follow target position d_p indicative of the follow target speed of the control target is close to zero.
The two conditions denote that the difference between the follow target position d_p and the position x to be controlled is maintained to be not large even in the defect period. To realize it, also in the defect period in which the position error d_p_e is missing, it is necessary to properly generate a signal driving the control target, specifically, the actuator control signal cont to be inputted to the electricity-to-force converting block 3, as if the control in the defect-free region is performed.
B1. Problems of the First Conventional Art
To solve the problems of the first conventional art, for example, the low-frequency bandwidth compensation characteristic of the phase compensating block 2 may be set to a wide bandwidth. FIGS. 49A and 49B show, in the control system 301, open-loop characteristic in the case of strongly compensating the low bandwidth as compared with the open-loop characteristic (indicated as “normal” in the drawing) shown in FIGS. 44A and 44B. FIG. 49A shows a gain characteristic, and FIG. 49B shows a phase characteristic.
In the case of strongly compensating the low bandwidth, the sensitivity characteristic of the control system 301 is as expressed by graphs of FIGS. 50A and 50B. FIG. 50A shows a gain characteristic, and FIG. 50B shows a phase characteristic. The sensitivity characteristic shown in FIGS. 46A and 46B is indicated as “normal” in FIGS. 50A and 50B.
In a frequency band in which the gain increases in the open-loop characteristic as shown in FIG. 49A, the sensitivity characteristic can be lowered as shown in FIG. 50A. However, as shown by a portion A in FIG. 50A, there is a problem such that the gain characteristic is raised near the crossover frequency. Further, as shown in FIG. 49b, a phase margin decreases. Consequently, when the low frequency band compensation is performed strong, stability of the position control deteriorates, and a problem such that the position x to be controlled oscillates easily.
In association with the low-frequency compensation, to solve a problem caused by strong compensating the low-frequency band of the control system, it is sufficient to add a stabilization compensator for advancing a phase in the phase compensating block 2. For example, the order of the phase compensation characteristic F(s) is increased. In an actual control system, in many cases, the phase compensation block 2 is formed in an LSI and defined by a filter of a predetermined order, and design tolerance is low. It is therefore difficult to arbitrarily increase the order of the phase compensation characteristic F(s).
B2. Problems of Second Conventional Art
The second conventional art is a technique effective when a disk physical distortion amount is small. However, it is feared that a problem occurs when the disk physical distortion amount is equal to or larger than a specification limitation of a disk. The problem will be described hereinafter.
The follow target position d_p which changes according to a disk physical distortion has a sine-wave-shaped periodic pattern synchronized with rotation of the disk. To make the position x to be controlled follow the follow target position d_p, the waveform of the actuator control signal cont also has a periodic waveform pattern of a sine wave shape synchronized with rotation of the disk. When the disk physical distortion becomes large, the amplitude of the periodic waveform pattern of the actuator control signal cont becomes large.
On the other hand, the actuator control signal cont becomes a constant value during the defect period, so that the actuator control signal cont shifts from the predetermined value to the periodic waveform pattern at the exit of the defect. In theory, therefore, before and after the defect exit, values of the actuator control signal cont do not coincide with each other. The larger the amplitude of the periodic waveform pattern of the actuator control signal cont during the defect free period is, the larger the mismatch becomes.
Since the sensitivity of a change in the position x to be controlled to the actuator control signal cont is determined by the electricity-to-force converting block 3 and the actuator mechanism block 4, the larger the mismatch before and after the defect end of the actuator control signal cont is, the larger the position error at the exit of the defect becomes.
When the increased position error exceeds a dynamic range of the position control in the control system 302, a problem such as a focus error in the case of the focusing control or pull-in to a neighboring track in the tracking control occurs.
The problem will be described concretely by using an analysis result by a simulation. The case of applying the control system to tracking control of a CD player will be described as an example. FIGS. 51A to 51D are graphs showing operation of the tracking control performed by the control system 302 and results of analysis made on assumption that a CD disk having an eccentricity amount of ±70 μm is used and a portion of the radius of 24 mm as the innermost part of a region in which recording tracks of a disk are formed is reproduced at a linear velocity of 1.2 m/s. The length of the defect region is assumed to be 3 mm. In an actual CD disk, however, the length of the defect region hardly exceeds 3 mm.
FIG. 51A show a change with time in the follow target position d_p, which corresponds to the eccentricity amount of the disk. In the simulation, the eccentricity pattern indicative of the eccentricity amount of the disk has a sine wave having an amplitude of ±70 μm and a frequency of about 8 Hz. In order to perform a simulation under the worst conditions, a case that a defect region exists in a position where the speed of the follow target becomes the highest, that is, near the point at which an eccentricity pattern of a sine wave shape crosses zero is assumed. In FIG. 51A, the time base around the zero cross of the downward pattern is enlarged, so that the eccentricity pattern has a linear waveform which descends to the right.
FIG. 51B shows the waveform of a residual error, FIG. 51C shows the waveform of the actuator control signal cont, and FIG. 51D shows the waveform of the defect detection signal DEFECT. As shown in FIG. 51D, the defect period starts around time at which the eccentricity pattern shown in FIG. 51A crosses zero, and the defect period is about 2.5 msec. It is understood from FIG. 51C that, during the defect detection signal DEFECT is at the “H” level, the actuator control signal cont is constant and the position control signal p_cont just before the entrance of a defect is held. The reason why the actuator control signal cont increases slightly at the time corresponding to the entrance of the defect is that the actuator control signal cont at the time prior to the time corresponding to the entrance by a delay time in the low-pass filter 100 is held.
According to the second conventional art, as shown in FIG. 51B, the absolute value of the residual error in the defect period increases like a quadratic function and increases as the defect region becomes larger. In the example, the residual error at the exit of the defect is about 0.75 μm. Considering that the limit point of pulling the position to be controlled into a neighboring track is at a half track pitch 0.8 μm in a CD, the size of the residual error causes unstable defect compensation.
According to the second related art as described above, in the case where the disk physical distortion such as eccentricity is large, in other words, in the case where a change in the follow target position d_p is large, the residual error becomes large at the exit of the defect, and there is a first problem such that continuity of the control may deteriorate.
Further, an influence of noise component included in the actuator control signal cont exerted to the defect compensating operation in the second conventional art will be described. Generally, the position error d_p_e includes high-frequency observation noise as compared with the disk rotation frequency, so that noise is added also to the phase control signal p_cont as an output of the phase compensation block 2 to which the position error d_p_e is inputted. Consequently, noise is added also to the actuator control signal cont in the defect-free period. The amplitude of the noise depends on the amplitude of noise added to the position error d_p_e.
In the case where noise added to the actuator control signal cont is random noise, an influence is hardly exerted onto the position x to be controlled. As described above, the actuator mechanism characteristic G(s) functions as an integrator expressed by a secondary model using a mass, a spring, and a dashpot. Consequently, even random noise is added, an integral becomes zero in the steady state, and an influence of random noise is very small.
At the entrance of the defect, however, there is no guarantee that the integral of noise until then in the actuator mechanism block 4 is zero, and the integral may include a DC component. In the case where the integral of noise has a DC component, it is equivalent to the case that the DC component acts as a pulse-shaped disturbance on the actuator during the defect period. There is the possibility that such a phenomenon causes a error in the speed of the actuator and an increase of the position error d_p_e.
In recent years, the phase compensation block 2 is constructed by an LSI (large-scale integrated circuit) which operates at a predetermined sampling frequency. In many cases, a digital processing is executed on the inside of the phase compensation block 2. In this case, noise including the sampling frequency is added to the position control signal p_cont. When such noise is integrated in the actuator mechanism block 4, the integral becomes a DC component and, moreover, its length may become a few times as long as the sample period.
To clarify the problem, an analysis result of a simulation of the tracking control of a CD is shown in FIGS. 52A to 52D. The graphs of FIGS. 52A to 52D show, as an example of the case of applying the control system 302 to tracking control of a CD player, a result of analysis in the case where there is no disk eccentricity, the length of the defect region is 3 mm, and reproduction is performed at a linear velocity of 1.2 m/s. As an example, it is assumed that the phase compensation block 2 is constructed by a digital circuit operating at a sampling frequency of 80 kHz, the amplitude of noise is 0.4 V, the width of noise is 25 μsec (corresponding to two sampling periods), and noise is added just before the entrance of a defect. Analysis of the function of the defect compensation with respect to this case is equivalent to analysis of the function of the defect compensation with respect to the case a pulse obtained as an integral of noise is added to the actuator control signal cont. The pulse has, concretely, an amplitude of 0.4 V during the period of the two sampling periods until the rising edge of the defect detection signal DEFECT, that is, at 25 μsec.
FIG. 52A shows the follow target position d_p. In the simulation, the eccentricity amount of a disk with respect to the follow target position d_p is zero. FIG. 52B shows the waveform of a residual error, FIG. 52C shows the waveform of the actuator control signal cont, and FIG. 52D shows the waveform of the defect detection signal DEFECT. As shown in FIG. 52D, the length of the defect period is about 2.5 msec. As shown in FIG. 52C, during the defect detection signal DEFECT is at the “H” level, the actuator control signal cont holds the actuator control signal cont in the defect-free period, that is, zero as the value of the phase control signal p_cont.
As shown in FIG. 52B, the absolute value of the residual error in the defect period increases like a quadratic function and increases as the defect region becomes larger. In the example, the residual error at the exit of the defect is about 5 μm, which is a value far exceeding the half track pitch of 0.8 μm in a CD as a limit point of pulling the position of the control target to a neighboring track, so that defect compensation becomes unstable.
According to the second conventional art as described above, the residual error at the exit of the defect increases due to noise added to the actuator control signal cont, and there is a second problem such that continuity of the control may be lost.